(1) Field of the Invention
The present invention relates to a method of treating a photosensitive printing plate. More particularly, the invention relates to a method wherein a photosensitive lithographic plate comprising a metal base is subjected to imagewise exposure, development and then burning-in treatment.
(2) Prior Art
Lithography is a printing method that takes full advantage of the fact that water and an oil are essentially immiscible with each other. The surface of a lithographic plate has a water-accepting, oily ink-repelling region and a water-repellent, oily ink-accepting region; the former being a non-image-forming region and the latter being an image-forming region. Therefore, a photosensitive composition used for making the lithographic plate must repel water and accept an oily ink after the image formation.
The photosensitive compositions used in the preparation of lithographic plates are classified into negative working and positive working ones. As positive working photosensitive compositions, those comprising an o-quinone diazide compound have been used widely.
The positive working photosensitive lithographic plates are prepared by applying an o-quinone diazide compound alone or a mixture thereof with an alkali-soluble resin such as a novolac phenolic resin or cresol resin to a metal or plastic base. When the photosensitive lithographic plate is exposed to actinic light though a positive transparency, the o-quinone diazide compound in the exposed region is decomposed to form an alkali-soluble compound, which is removed easily with an aqueous alkali solution to form a positive image. Thus, when a base having a hydrophilic surface is used, the hydrophilic surface is exposed in the region in which the o-quinone diazide compound has been removed with the aqueous alkali solution. Therefore, water is accepted and an ink is repelled in this region. On the other hand, the remaining, image-forming region is oleophilic and accepts the ink.
The negative working photosensitive compositions usually comprise a diazonium salt, azide compound or photopolymerizable compound. These compositions are applied to a base either alone or in the form of a mixture with an additive such as a suitable resin. When the base used has a hydrophilic surface, unexposed photosensitive composition is removed with a developer to expose the hydrophilic surface of the base. This hydrophilic surface accepts water and repels the ink. On the other hand, a photosensitive composition hardened by the exposure and left as an image in the development step is oleophilic and, therefore, accepts the ink.
The lithographic plate thus prepared is placed in an offset printing machine to obtain clear prints. By properly selecting the base and the photosensitive layer composition to be coated on the base of the lithographic plate thus preparing the photosensitive plate, tens of thousands of beautiful prints can be obtained. Particularly when an aluminum plate, which has been grained and treated by anodic oxidation, is used a hundred thousand clear prints can be obtained.
However, in some cases it is desirable to obtain much more prints from a single printing plate. In such a case, a photosensitive lithographic plate comprising a metal base plate such as an aluminum or zinc plate is exposed and developed by an ordinary method and then heated to a high temperature to effect a so-called burning-in treatment and thereby strengthening the image-forming part.
By effecting the burning-in treatment, the number of prints obtainable from a single lithographic plate can be increased to several times that obtainable when the burning-in treatment is not employed.
Further a special printing ink can be used which contains a large amount of a component in which the image-forming components on the printing plate are soluble such as an U.V. curing ink or low temperature drying ink. With this ink, a sufficient number of printed sheets can be obtained by using the lithographic plate subjected to the burning-in treatment, since the resistance of the image-forming part to the solvent is improved remarkably. However the number of the printed sheets obtained by the aforementioned arrangement is reduced drastically as compared with the number of printed sheets obtained by using ordinary ink with a plate prepared by an ordinary method including no burning-in treatment, due to the dissolution of the image-forming part.
However, by the burning-in treatment, the hydrophilic property of the non-image-forming part of the printing plate (i.e. a part in which the hydrophilic base surface has been exposed by the development) deteriorates while accepting, the printing ink to stain the background of the print. But, when the degree of heating is reduced, so as not to cause background stain, the burning effect is not obtained and the image-forming part is not reinforced. Thus, a counter-etching treatment is indispensable for preventing the non-image-forming part from staining from the burning-in treatment. Various processes for counter-etching, both prior to and/or after the burning treatment, have been proposed.
For example, an aqueous solution of a fluoride such as hydrofluoric acid, borofluoric acid or hydrosilicofluoric acid has been used as an agent for removing stains in the non-image-forming part and recovering the hydrophilic property after the burning treatment. However, these fluorides are poisonous substances and they pose environmental pollution problems.
Further, when the above described counter-etching treatment is effected, the metallic surface of the base is corroded, consequentially it easily scratches and abrasion resistance is reduced. As a result, the water retention ability of the non-image-forming part is lost over the course of the printing, background stain is caused, and it becomes intolerable to printing.
A process for preventing the reduction of the hydrophilic property in the non-image-forming region, prior to the burning-in treatment, is disclosed in U.S. Pat. No. 4,294,910 which comprises treating the plate with an aqueous solution of an organic sulfonic acid salt, such as a sodium alkylnaphthalenesulfonate or sodium alkyl diphenyl ether sulfonate or lithium nitrate, prior to the burning. Among them, the aqueous solution of the organic sulfonic acid salt, such as the sodium alkylnaphthalenesulfonate or sodium alkyl diphenyl ether sulfonate, are unsuitable for use in the spray circulation type treating machine now generally used, since it has an extremely high foaming property. When said solution is used, the process cannot be automated. Further, when the plate is treated with a lithium nitrate solution and then subjected to burning-in treatment, sufficient prevention of background stains is impossible. U.S. Pat. No. 4,063,507 discloses a process wherein the plate is treated with an aqueous solution of subliming boric acid and its salt prior to the burning-in treatment and the plate is subjected to the burning treatment in the presence of said compounds. However, there is insufficient prevention of background stains by this process. With this process background occurs easily depending on the conditions of washing with water, and gumming is effected after the burning-in treatment. This is particularly true when the water wash is insufficient or a desensitizing gum comprising a dextrin having a low hydrophilyzing power is used as a lithographic finisher. A process, wherein the plate is treated with an aqueous solution of an organic substance, such as gum arabic, cellulose ether or polyacrylic acid and/or an aqueous solution of a water-soluble inorganic salt such as a boric acid salt, phosphoric acid salt or sulfuric acid salt or an alkali metal or alkaline earth metal halide, prior to the burning-in treatment, is disclosed in Canadian Pat. No. 1084758. However, this process also has defects in that background stains are not always completely prevented. When gum arabic, or an aqueous solution of a water-soluble polymer such as polyacrylic acid is used, the image-forming region barely accepts ink (so-called image-blinding). Further, a process wherein the plate is treated with an aqueous solution of an amine having a carboxyl group, such as ethylenediaminetetraacetic acid or hydroxyalkylethylene-diaminetriacetic acid or a salt thereof, prior to the burning-in treatment is disclosed in U.S. Pat. No. 4,355,096. Likewise, in this process, the effect of preventing background stain is still insufficient.